Field
The present disclosure relates generally to wireless communications, and more particular, to systems and methods for controlling the transmit power in a wireless communications device.
Wireless communication systems are designed to allow multiple users to share a common communications medium. Numerous multiple-access techniques are known in the art such as Code Division Multiple Access (CDMA), time division multiple access, frequency division multiple access, as well as many other similar multi-access techniques. The multi-access concept is based on channel allocations that separate communications among multiple users. By way of example, in CDMA communication systems, channel separation may be achieved by transmitting each signal with a different code that modulates a carrier, and thereby, spreads the signal over the entire frequency spectrum. The transmitted signals can be separated in the receiver by a demodulator using a corresponding code to de-spread the desired signal. The undesired signals, whose codes do not match, contribute only to noise.
The ability of a receiver to separate the transmitted signal from the noise may be measured by the Signal-to-Noise Ratio (SNR). The average signal transmission power may be controlled at the transmitter to achieve a specific SNR at the receiver in accordance with the quality of service requirements. Typically, a higher SNR is required to achieve a higher data rate that might be required for a video or other high bandwidth transmission. A higher SNR may also be required if the transmitter is far away from the receiver. In either case, improvements in the SNR may be achieved by increasing the average signal transmit power at the transmitter. Unfortunately, this increase may result in signal power peaks that exceed regulatory limits, such as those imposed by the Federal Communications Commission in the United States. This may occur in CDMA communication systems where each user simultaneously transmits over the same frequency spectrum. Multilevel modulation and transmit pulse shape filtering further increase the probability that the peak transmit power may exceed the regulatory limits.
One way to increase the average transmit power while maintaining the peak transmit within the regulatory limits is to clip the signal when the instantaneous transmit power is above a certain threshold. However, clipping a signal can significantly increase the out-of band emissions, as well as introduce distortion in the signal. Out-of-band emissions are also regulated, and therefore, may significantly limit the use of clipping to reduce peaks in the signal power.
Accordingly, there is a need in the art for optimizing the average transmit power to maintain a desired quality of service, and still meet the regulatory limits for both peak transmit power and out-of-band emissions.